Let’s say http://example.com/happy/ was displaying a 404 page to the user trying to access it, but when checking the response code using an HTTP status code checker, it returned an HTTP 200 response. This is a prime example of a Soft 404 error, as the HTTP response code is indicating to search engine robots that the page exists and should be crawled. However, there is no content on the page that’s returned by the server.
I run your 404 error url through an HTTP status code checker httpstatus.io, to assess which status codes was are returned. It gives you insight to what is going on.
Another tool that you can use to check the HTTP response codes returned by your webpages is Tools I Seek.
You can also read more in this article What Are ‘Soft’ 404 Errors and Will They Affect My Rankings?
You may also check out these resources for WordPress Yoast SEO.